1. Field of the Invention
This invention relates generally to color presentation by different devices--for example different printers, cathode-ray tubes (CRTs), and liquid-crystal displays (LCDs)--and more particularly to coping with divergent color gamuts of such color-presenting devices, when color specifications developed with one device are used to control color presentation on another device.
An exemplary application is in controlling color presentation by a printer, based on color specifications developed using a CRT computer monitor.
2. Related Art
A. Coexistent machine color spaces--The use of color monitors for computers has accelerated the need for color printers which can produce a satisfactory printout of what is displayed on the screen. As suggested above, this is one important instance of coordinating color appearances produced with two different physical systems.
With all such equipment, determining what constitutes a "satisfactory" color presentation--for instance, color printout on a printing medium such as paper--is often quite problematic.
Part of this problem arises from the subjective nature of color. Color is a sensation produced by combined effects of light, objects and human vision. A particular color or combination of colors may be appealing to one person while at the same time being offensive to another.
Another part of the "satisfactory"-color definitional problem arises from different color technologies used in computer monitors, color printers, and other color-presenting devices--such as broadcast or tape-recorded video. In general these technologies diverge dramatically.
For example, color presentation by CRT computer monitors and television sets is based on a color gamut defined by red, green and blue (RGB) CRT intensities. Color presentation by printers such as inkjet printers is instead typically based on a color gamut defined by cyan, magenta, yellow and black (CMYK) printed-page colorants.
The RGB color intensities of CRT screens are combined together in an additive way by mixing red, green and blue light rays from a first class of physical substances--namely phosphors--to form a first variety of different colors. The CMYK components of color inks, a second and entirely different class of physical substances, are applied to media in different combinations in a subtractive way to form a second variety of different colors; and the three chromatic elements CMY are only nominally the complements of the RGB intensities.
Various different color-management techniques have been used to provide some form of coordination between, for instance, the colors viewed on a computer monitor and the colors printed by a specific printer using a given ink formula on a particular type of printing medium. Modernly, most such control regimes are implemented in computer software or device firmware, or both.
Some color-matching technology has been incorporated into printer drivers, and in some such cases partially into accompanying lookup tables. A driver provides a translation interface between--on one hand--a particular computer operating system, and/or application software running in the computer, and on the other hand a color printer which acts as a hardcopy output device.
Portions of this document refer to an "input device" or "source device". By this is meant a device in reference to which a set of color-image specifications has been developed--not necessarily a device within which specifications literally originate.
Quite commonly, for example, a person watching a CRT screen (here serving as the input device) creates or modifies specifications for the colors, as seen on the screen, of various image portions, respectively. A printer or other device may be used instead of a CRT as the input device, even though naturally CRT screens are in general much faster and so in most cases preferred for this purpose--and conversely a CRT may be an output device.
It will be clear to readers who are skilled in this field that the CRT screen or other input device is not literally the source of the specifications. Rather, the source of the specifications is usually the person, typically in interaction with some apparatus or computer program--often a so-called "applications" program.
Such apparatus or program receives the person's commands, translates them into display-device signals and causes them to be displayed on the CRT screen. The physical properties (or so-called "characterization") of the CRT enter into the determination of how the specified colors will appear.
Once created or modified, or both, specifications are commonly directed into control systems for operation of another device--the "output device" or "destination device"--often a printer. Thus in many cases both the "input device" and "output device" in common are recipients of the specifications, in the form of data, electrical signals etc.--and the differences between physical properties or "characterizations" of the two devices in general give rise to differences between the resulting color appearances.
Terms such as "source device" or "input device" in this document thus encompass color-presenting devices which may be used by a person in conjunction with developing color-image specifications. These terms also, however, encompass devices that actually do serve as the source of the specifications.
An instance of this which is of particular interest is an applications program or system which leaves--as some do--the CRT signals or data essentially unaltered. In such a case the information in substantially the same form as used to control the CRT is treated as, or may be regarded as, the color specifications.
B. Pipe-through systems--In designing control systems for early apparatus, a classic assumption was sometimes used that the red, green and blue (for example) of one system were compatible, or consistent, with the red, green and blue complements of the cyan, magenta and yellow (for example) of another system. These assumptions are grossly incorrect, but had several interesting and useful properties.
First, the assumptions and resulting conversions were simple and required little understanding of relationships between the different color-production mechanisms used in the different systems. In particular these assumptions sidestepped all inquiry into the color gamuts respectively available with the two systems, and so into the relationships between those gamuts.
Therefore these assumptions were very easily implemented. In essence the RGB data were simply piped through to form CMYK data--but with elementary conversions based on the previously well-taught oversimplifications of so-called "complementary" colors; and also with some side calculations to derive black-ink (K) quantities.
Second, seen from the more-modern perspective of gamut relationships, these assumptions had the technical or theoretical benefit of in essence forcing both gamuts into concurrence, thus completely using both color spaces. They perfectly superposed the source- and output-device gamuts, thus producing colors with--potentially--a full range of vividnesses and lightnesses.
Third, viewed from a simple perceptual standpoint, these assumptions did produce color presentations for output devices (e.g., usually printers) that were usually distorted in relation to the colors presented by the input devices (e.g., usually CRTs). The relationship was generally quite poor--suffering from severe errors in hue and chroma, usually obscured by even more severe errors in lightness.
Because apparatus users found these lightness errors unacceptable, system designers next proceeded to incorporate lightness/darkness adjustments into the otherwise piped-through signals. These adjustments were sometimes labeled with such words as "lightness" and "darkness".
Sometimes they were instead called adjustments in "contrast" and designated by the corresponding parameter ".gamma." (a lower-case Greek letter "gamma")--a classical variable in photography, photolithography and photometrics. In either case these adjustments were often actually implemented in the classical manner as .gamma. slopes in a logarithmic response space.
In any event, lightness/darkness corrections generally took the form of separate adjustments--generated painstakingly through trial-and-error--to red, green and blue input signals. Through these efforts, lightness as presented by an output device could be brought into an acceptable range of relationships with that presented by an input device.
Another result, however, as the residual of this process was to reveal the theretofore-unaddressed deviations in hue and chroma. With lightness errors under relatively better control, these chromatic errors became far more conspicuous--and they are much more difficult to master.
C. Overlapping gamuts--In later apparatus some effort was addressed to controlling the relationship between these perceptual chromatic elements (hue and chroma) of input- and output-system colors. This effort generally took the form of attempts to match the appearances of the two.
This step entailed analytically characterizing the two devices--e.g., CRT and printer--in some common perceptual space. At this point the physical reality of the differences between the two sets of colors and the two device gamuts came out, disclosed by the perceptual frame of reference.
Now for the first time it became meaningful to consider the matter of dealing with gamut divergences. In general the gamuts of any two different devices fail to be congruent, and in particular often are overlapping--in all three dimensions of color space.
Thus typically each of the two gamuts extends to more-vivid colors, in some parts of the lightness ranges and some parts of the hue spectrum, than does the other gamut. Moreover, typically one gamut extends to lighter or to darker colors, or to both lighter and darker colors, than does the other gamut.
This overlap phenomenon implies, for example, that some colors which can be displayed on a CRT simply cannot be printed on a printer. It also has a converse implication (which will be introduced shortly)--but the unavailability of printer representations for some CRT-representable points is a particularly demanding matter.
It is demanding because color specifications developed by a person operating a CRT will in general include data points that are outside the printer gamut--but yet must be processed in some way. Thus system programmers, whether they wish to make color-relationship decisions or not, are forced to instruct systems what to do with those out-of-gamut points.
Faced with this necessity, system designers devised one or another scheme for translating the furthest-out-of-gamut CRT color into the interior of the printer gamut. A natural concern in specifying such a scheme is how to retain some degree of consistency in dealing with all colors--in other words CRT color-data points that are within the printer gamut, as well as different points that are outside it.
This idea of consistency is very important, particularly in presenting color images that include pictures, intended to appear realistic, of physical objects. Pictures of objects often derive a degree of realism through preservation of color relationships between colors of physically nearby image elements, such as for instance adjacent portions of subtlely-shaded clouds, or of the surfaces of curved objects. Another way to refer to preserving such "relationships" is to speak of retaining "information content".
A classical example of the relationship-preserving or information-content-retaining challenge has been denominated the "gum-ball problem". This phrase conjures the difficulty of realistically presenting a color picture of an object whose portrayed curvatures extend through a full range of values from flat-on to tangential.
As is well known, realistic portrayal of curvatures generally requires reproduction of a continuum of surface shadings--without which surfaces tend to appear, literally, flat. Attaining such realistic presentation is particularly difficult for a vividly colored object such as a gum ball.
The necessary shadings needed to realistically portray curvatures are especially demanding for vividly colored objects because these colors are near the peripheries of overlapping-gamut systems. In particular, often, these colors are within the source-device gamut but outside the destination-device gamut.
The difficulty arises from the need to somehow preserve as much as possible of the relationships between these many different shadings found in the source-device gamut--even when they are all outside the output-device gamut. The question then is how to represent, within the capabilities of an output device, relationships between many different vivid-color appearances that are all, by definition, outside those capabilities.
D. Gamut compression--One standard solution to these concerns heretofore has been to make a proportional adjustment to the chroma, or distance from the central lightness/darkness axis of the three-dimensional color-solid gamut, for all CRT-defined points. A common variant of this technique is proportional adjustment to both chroma and lightness--the latter being represented as position projected along that central axis.
The chroma-adjusting proportionality factor in such systems is less than unity, and is generally constant. Usually the proportionality factor is constant throughout the entire three-dimensional gamut; possibly in some systems, however, that factor is at least constant throughout each so-called "hue page", or defined grouping of hue pages.
(A hue page, as considered within any single specified color space, is a vertical plane extending radially outward from the central color-space axis of lightness/darkness, and representing the available variations of chroma and lightness that make up all the available colors of a selected single hue. As between different color spaces, the hue-page concept has important limitations which are discussed in section d! below.)
The selection of a constant proportion throughout the color space may be an intuitively satisfying choice, since its physical implication is in theory a mapping of all colors with no change in lightness or hue, only a change in vividness--and since, furthermore, at least in an analytical sense this vividness change is applied consistently to all the colors in the CRT gamut.
Thus the various colors retain nearly their original lightness and hue, and also retain their mutual relationships in a comparative way. For example, if a first color on the CRT screen is originally darker, bluer and more saturated than a second on the CRT screen, then in principle the resulting, first color printed on paper should likewise be darker, bluer and more saturated than the resulting second color printed on paper--and a like relationship should be exhibited for any selected pair of colors.
One variant of this proportional-adjustment technique is to incorporate accompanying lightness adjustments, or often lightness/chroma tradeoffs (which themselves may or may not operate on the same principle of proportionality). These adjustments are provided to avoid some intuitively unsatisfying results of proportionally adjusting chroma exclusively.
All such proportional-adjustment techniques, sometimes known as "gamut compression", nevertheless have drawbacks. One relatively minor problem is that the theoretically consistent character of the transformation is not in general observed for real systems; in other words, the vividness compression is sometimes perceptually nonuniform.
More serious and fundamental is the fact that gamut compression artificially restrains the vividness of almost all the colors printed. The only colors in the CRT gamut which are printed near the full vividness of which the printer is capable are (1) the CRT color which is furthest out of the printer gamut and (2) the near-neighbors of that CRT color.
Even that furthest-out-of-gamut color is, by definition, very significantly reduced in vividness from what is seen on the CRT screen--but this cannot be helped because the printer gamut at this point does in fact limit the available vividness. Everywhere else in the printer gamut, however, the capability of the printer to present saturated, vivid colors is essentially discarded.
Thus in essence all the colors in the CRT screen gamut are synthetically and systematically reduced to the lowest common denominator, so to speak, of the printer gamut. One result has been to create among many users a misapprehension that CRT screens are capable of displaying colors vividly whereas CMYK printers are not.
As to printers the contrary is true--but the colors which printers can print vividly are different colors from those which CRT screens can display vividly. This is the "converse implication" mentioned parenthetically above.
To state that converse implication directly: some colors which can be printed on a printer simply cannot be displayed on a CRT. Evidence of this fact heretofore was, in effect, inadvertently concealed from many users by the popular technology of color-matching and gamut compression.
Thus in inventing gamut compression to solve the earlier problems of lightness and hue shifts in printed colors, relative to CRT-displayed colors, system designers created a new problem: printed colors which failed to make use of the printer gamut and so appeared--in comparison with CRT displays--drab, lifeless, boring, and generally disappointing to users of color-presenting apparatus.
Gamut compression, while serviceable in many contexts, thus failed to provide a technique for presenting such colors in a manner that is both fully useful (retaining information content, to a reasonable extent) and perceptually vibrant.
An earlier passage in this discussion posed the problem of "how to represent, within the capabilities of an output device, relationships between many different vivid-color appearances that are all, by definition, outside those capabilities." To that expression of the problem may now be added the additional constraint that the desired representation should at the same time preserve, to the extent possible, the vividness of the original colors.
The gum-ball problem, after all, is easily solved in the limiting case of black-and-white pictures--or whenever vibrant color is of little consequence. The challenge resides in concurrently retaining the vividness of the bright colors used in making the balls.
E. Surface scaling--Another line of effort has focused more emphatically on this latter constraint. Unfortunately, as will now be seen, the result has been to sacrifice a large part of the information-preserving or relationship-preserving concern.
Here the technique entails generally maintaining essentially unchanged the colors within the common portions of the input and output gamuts. Adjustments are applied exclusively--or almost exclusively--to those specified colors which are within the source-device gamut but found to be outside the destination-device gamut.
These adjustments consist of mapping, or moving, all those specified colors to some points along the destination-device gamut surface. In this line of development, known as "surface scaling", various rules have been adopted by various workers for selecting the new point along the output-gamut surface to assign to each given out-of-gamut color.
At the outset it will be understood that considerable loss of information is necessarily inherent in any such exercise, for at heart it consists of mapping areas into lines. Nonetheless the effort is entirely meritorious because retention of vividness is very important.
In fact, for many users vividness of the final presented color is at least as important as, and sometimes more important than, the retention of intercolor relationships and thus information conveying, e.g., spatial curvatures of objects portrayed. A major issue, then, is the extent to which the appearance, or even illusion, of seeming to retain such relationships can be sustained.
One straightforward kind of surface scaling is mapping colors to the output-gamut surface at constant lightness--in other words, displacing each out-of-destination-device-gamut point along a horizontal (constant-lightness) line to the intersection of that line with the gamut boundary. Intellectually this may seem the optimum arrangement, since it preserves lightness, and operationally may be appealing in that it is very simple to implement.
Unfortunately, however, because of the relative geometries of gamut boundaries, this approach tends to preserve relatively very small fractions of the relationships between out-of-destination-gamut colors. In other words this approach typically retains very small fractions of the information needed to distinguish different parts of objects.
Consideration of typical device-gamut shapes, particularly within hue pages, reveals why this is so. Device gamut boundaries as represented in perceptual spaces, in the usual lightness-vs.-chroma hue-page geometry, typically lie neither near-horizontal nor near-vertical but rather middlingly in between.
Hence a horizontal (along-constant-chroma) displacement of out-of-destination-gamut points approaches the destination-gamut boundary along an acute angle. The result of using this particular form of area-to-line mapping is to map a very long, horizontal line (within the out-of-gamut areas) into each point on the destination-gamut surface.
Similar objections may be lodged against a competing candidate mapping--namely, along vertical (constant chroma) lines. While perhaps not as often sacrificing as much information content in general, such a mapping tends to collapse multiple values of lightness onto each gamut-boundary point.
This particular sort of information deformation is particularly troublesome: first, it flattens apparent shapes of curved surfaces. Second, it also draws attention to itself by introducing relatively severe lightness shifts per se--and, as previously suggested, shifts in this color dimension are particularly noticeable and objectionable.
Mindful of these relationships, some workers choose instead to map each out-of-gamut color point to the nearest point on the gamut boundary. For much of the out-of-gamut area this amounts to displacing the color points along a normal to the boundary, which--considering typical overlapping-gamut geometries--can be seen to minimize the length of displacement.
In this way, still for much but not all of the out-of-destination-gamut area, this mapping geometry tends to maximize the extent of relationship/information retention. In certain areas of the hue page, however, the mechanism leading to this benefit tends to break down.
Those include, first, the areas in which there is no normal to the gamut boundary, and second the areas which are adjacent to those areas. Study of hue-page graphs for typical overlapping-gamut situations will reveal that these conditions can usually obtain only in relatively small hue-page regions.
These are the regions near the maximum-saturation point and the lightness/darkness-extremum points for the destination gamut. From the localization and smallness of these effects it might be supposed that they are correspondingly of little consequence.
To the contrary, as a practical matter it is found that the quality of mapping in these areas is particularly important to the user. Areas near the maximum-saturation point are of particular concern to users generally, as the earlier presentation of the "gum-ball problem" makes evident.
Mapping performance in areas near the extrema of darkness and lightness implicates the capability of an image-presentation system to preserve image information or detail only in very dark and very light parts of the image. As such, image appearance in these selected areas may seem an obscure criterion.
Such a criterion may perhaps be of greatest concern only to relatively more-discerning users, but of these there are plenty. Furthermore, classical evaluations of image-presenting technologies from oil painting through photography and photolithography have concentrated attention upon these criteria.
With this in mind, one may go on to notice an important phenomenon of surface scaling by application of any algorithm that maps to the nearest surface point. In the out-of-gamut regions near the three extremum points, those extremum points tend to act as lightning rods, collecting all the colors from diverging, sector-shaped regions outside the destination gamut.
It is fair to conclude that surface-scaling techniques of the to-the-nearest-boundary-point type leave considerable room for improvement with respect to image-presenting quality in these critical regions of the color space.
F. Data storage and processing requirements--Heretofore the amount of data needed for useful characterization of hue pages in machine or perceptual color spaces has bordered on the massive.
In some cases characterizations have taken such forms as, for instance, point-by-point perceptual chroma and lightness values corresponding to RGB data triplets or other machine-space data. Typically these values are tabulated for each point in a grid covering the entire interior of a hue page (or other color-space element) of interest.
In other cases characterizations have taken the form of point-by-point descriptions of upper and lower gamut boundaries for each hue page of interest. Here the amount of data required is less than in the full-interior characterization just mentioned, but still unwieldy.
Even with such multipoint characterizations, considerable operating time typically must be devoted to interpolation, in three color-space dimensions, between the table entries. Thus operating-time cost is added to the cost of data storage.
In still other cases perhaps there has been put into use, for characterizing chroma and lightness within a hue page, some multiterm polynomial expansion that satisfies boundary values for each hue page of interest. This may be an improvement, but has some tendency merely to replace near-massive data storage with near-massive data processing--a substitution that saves disc space only at the cost of time and throughput penalties.
For some purposes, detailed and accurate lookup tables, or calculations good to three or four places, seem serviceable and in some cases perhaps even unavoidable. A methodology, however, heretofore has been severely needed for at least mitigating this brute-force approach to machine-and perceptual-space characterizations, wherever extremely high precision or accuracy is not fundamentally necessary.
G. Summary--Thus there has been heretofore a need for a refined color-management technology which somehow permits use of the full color-saturating capability of printers. It is desirable that such a technology at the same time preserve at least some of the acknowledged benefits of color-matching, and of its perhaps most-popular implementation, gamut compression.
For example, again with respect to CRT/printer systems, the related art provides no system for printing well-saturated color in conjunction with:
at least approximate preservation of the lightness relationships in a CRT-displayed image; or PA1 preservation of what may be called "information"--meaning the discrimination between initially adjacent colors, with at least some preservation of the hue and of the relative, if not absolute, lightness and vividness magnitudes--in a CRT-displayed image. PA1 at least approximate preservation of the lightness relationships in a corresponding image produced by a source color-presenting device; or PA1 preservation of information in the source color-presenting device. PA1 determining the position of the color specification within a first color gamut, PA1 superposing, at least approximately, elements of that color gamut and a second color gamut, and PA1 using the at-least-approximate superposition, and using the color-specification position, to define a new color specification for the same portion of the image. PA1 interpreting relative-position values of lightness and chroma, within a display-device hue page that is part of a display-device gamut in a perceptual space, as also being relative-position values of lightness and chroma within a corresponding hue page that is also part of the printer gamut in the same perceptual space, and PA1 using these relative-position values of lightness and chroma, within the printer gamut, to derive printer signals. PA1 based upon the selected specifications in terms of a hue page that is part of the display-device gamut and display-device color space, determining corresponding values of hue, lightness and chroma, expressed in terms of absolute position within a display-device hue page in a perceptual color space, PA1 finding hue-page characterizations for the display-device and printer hue pages (and particularly for the gamut boundaries of those pages), expressed in that same perceptual space, for the determined hue--also expressed in that perceptual space, PA1 using the found display-device (gamut) characterization, and also using the found absolute-position values of lightness and chroma expressed in the perceptual space, to find corresponding first intermediate values of lightness and chroma, expressed in terms of relative position in the same perceptual space within the display-device gamut, PA1 employing the found printer (gamut) characterization, and also the first intermediate relative-position values of lightness and chroma, interpreted as also being relative-position values of lightness and chroma still in the same perceptual space within the printer gamut, to find second intermediate values of lightness and chroma expressed in terms of absolute position in that perceptual space within the printer gamut, and PA1 based upon the hue and the second intermediate relative-position values, all expressed in the same perceptual space, obtaining new color specifications expressed in terms of the printer gamut and a color space used for operation of the printer, for the particular image portion. PA1 i. absolute values in input-device space translated into perceptual space, PA1 ii. absolute to normalized values, PA1 iii. input-device renamed to output-device values, PA1 iv. normalized to absolute values, PA1 v. perceptual-space to output-device space, and finally PA1 vi. driving the output device. PA1 interpreting relative-position values of lightness and chroma, within a display-device hue page that is part of the display-device gamut in a perceptual space, as also being relative-position values of lightness and chroma within a corresponding printer hue page within the printer gamut, in that same perceptual space; PA1 employing these relative-position values within the printer hue page to derive printer signals; and PA1 then applying the printer signals to control printing by the printer. PA1 receiving or developing, for a particular image portion, display-device signals for controlling red, green and blue intensities for each image portion, in operation of the display device; PA1 performing on the display-device signals a transformation that has the effect of performing these substeps: PA1 then applying the printer signals to control printing of the particular image portion by the printer.
Now couching these same problems in broader terms, the need is for--in operation of a destination or target color-presenting device
In principle the source and destination devices may be two different kinds of printers, or two different kinds of computer monitors (e.g., one an LCD screen), or the source device a printer and the target a display screen, or one device a broadcast or tape-recorded video monitor and the other a lithography system, etc.
It will be recalled that the early, classical system of, in essence, directing source-device signals to output-or destination devices did have one technical advantage: the gamuts of the two devices were fully and perfectly superposed. Satisfying this condition ensures complete gamut usage (including vivid colors) and complete information preservation.
It is accordingly desirable in a modern system to restore some such relation--but not perceptually willy-nilly as in the early systems. Rather instead gamut superposition should be effected in a manner that somehow incorporates a perceptual frame of reference, to provide better hue control and some more-acceptable degree of lightness control than in those early systems.
In addition it is desirable to provide methodology for avoiding heavy usage of memory and processing time where not really essential. This is particularly important as to transformations within hue pages.
As can now be seen, important aspects of the technology which is used in the field of the invention are amenable to useful refinement.